An easy way to fix this particular problem would
perhaps be to just change the description of the old format (since that
is just a part of the doc string I assume):
- (old): (sit-for SECONDS &optional MILLISECONDS OLD-NODISP)
That would not help argument highlighting (the second declaration
won't be used to detect argument references in the docstring), and you
will need to add a section explaining what OLD-NODISP do. As it is
now, it is clear that MILLISECONDS is an additional argument that just
happens to be between SECONDS and NODISP.